The invention relates to a methods and apparatus for determining an integrity risk for use in a satellite referencing system.
Satellite reference systems, such as the European Galileo Satellite Navigation System (in the following called Galileo System or abbreviated Galileo), currently under construction, have a satellite system comprising a plurality of satellites and an earth-fixed receiving equipment system connected with a central computing station, as well as utilization systems which evaluate and utilize the satellite signals transmitted by the satellites. The satellite signals can be used for position indicating or navigation.
The aviation community is interested in a suitable integrity concept for satellite reference systems. The current integrity concept for Galileo provides the computing of an integrity risk at an alarm threshold value for four different conditions and adding the weighted contribution to the entire integrity risk. When this risk is above a permitted integrity risk, a user is informed by way of an alarm. This concept offers the least conservative assessment of the integrity risk while the continuity and the availability are the highest. However, it is computed by a non-standardized procedure. The integrity concept of Galileo is described in detail in International Patent Document WO 2005/088332 A2.
It is therefore an object of the present invention to provide improved methods and apparatus for determining an integrity risk for satellite reference systems.
One aspect of the present invention now involves permitting a mapping of parameters used in the Galileo integrity concept on the parameters used in the SBAS (Satellite Based Augmentation System). Such an approach meets the wishes of the aviation community which is asking for an integrity concept also for the Galileo system that is identical with the concept used for the SBAS.
One example of an SBAS System is the WAAS (Wide Area Augmentation System). By way of the WAAS, correction signals can be transmitted which permit a more precise position indication. Exemplary embodiments of the present invention reference the SBAS and WAAS concepts and equations.
An integrity risk is the probability that a computed position error during an arbitrary continuous period exceeds a determined threshold value (the “alert limit”) and the user is not informed within a determined time period.
A false alarm, thus, an erroneously triggered alarm, always occurs when the estimated SISE (signal in space error) is greater than a defined threshold value and the actual SISE is smaller than the defined threshold value.
The term “signal in space” originates from the task of a satellite navigation system of distributing information via signals in space in order to permit a position determination. In the Galileo System, each satellite emits four ranging signals which, as a function of the respective signal and service, contain ephemeredes, clock parameters, integrity information and other data.
An SISE (signal in space error) is the maximal effect of the error on the distance between the satellite and the user that the signal has when leaving the phase center of the satellite antenna.
A prediction of the precision of the navigation signals is called a “signal in space accuracy” (SISA). The SISA indicates a prediction of the minimal standard deviation of a Gaussian distribution overbounding the distributions of the SISE of all possible user positions in the visibility range of the satellite in the event that no systematic errors occur.
In this case, “bounding” is defined as follows: a distribution of a random variable A is “bounded” by a distribution of a random variable B when, with the probabilities A<−L and for A>+L, the latter is smaller than the sum of the probabilities for B<−L and for B>+L, for all L≠0.
According to an embodiment, exemplary embodiments of the present invention relate to a method of determining an integrity risk in the case of a satellite reference system, which comprises the following steps:                detecting Galileo parameters according to the Galileo integrity concept;        mapping the Galileo parameters onto SBAS parameters; and        determining the integrity risk for the satellite reference system according to the SBAS integrity concept using the SBAS parameters.        
This approach has the advantage that the standardized WAAS protection level can be used. An integrity concept is therefore made possible, for example, for the Galileo System, which integrity concept is based on the protection level defined for the SBAS.
According to one aspect, the step of detecting may include a detection of the Galileo parameters “signal in space accuracy” (SISA), “signal in space monitoring accuracy” (SISMA) and the probability of a “ranging signal failure”. The step of mapping can comprise a computing of a new “signal in space monitored accuracy” (SISMdA) parameter, and the step of determining can comprise using the new SISMdA parameter as a timing and orbiting fraction of the user differential range error (UDRE) in the protection level equations standardized for the SBAS.
Furthermore, for the Galileo integrity concept, a “range error” distribution of faultless satellites can be bounded by a Gaussian distribution with a “zero” mean value and a “signal in space accuracy” (SISA) standard deviation, and a distribution of the difference between the estimated “signal in space error” and the true “signal in space error” can be bounded by a Gaussian distribution with a “zero” mean value and a “signal in space monitoring accuracy (SISMA) standard deviation.
A Gaussian function with a standard deviation SISMdA and a “zero” mean value can be bounded by a weighted sum of the faultless and the faulty distribution of a satellite.
A “range error” distribution of a faulty satellite may be a Gaussian distribution with a standard deviation SISMA and a mean value, which is a prefactor which reflects a permitted error alarm rate multiplied by the square root of the sum of the square of SISMA and the square of SISA.
According to another aspect, two-frequency “ranging signals” can be used, and an ionospheric contribution of the WAAS equations can be correspondingly adapted to the Galileo equations.
According to a further aspect, the present invention relates to a computer program for the implementation of a method according to an embodiment of the present invention and to a computer program product containing a machine-readable program carrier on which the computer program can be stored in the form of electronically and/or optically readable control signals.
According to another aspect, the present invention relates to an apparatus for determining an integrity risk in the case of a satellite reference system, having the following characteristics:                a device that detects Galileo parameters according to the Galileo integrity concept;        a device that maps the Galileo parameters onto SBAS parameters; and        a device that determines the integrity risk for the satellite reference system according to the SBAS integrity concept using the SBAS parameters.        
Additional advantages and application possibilities of the present invention are indicated in the following description in connection with the exemplary embodiments illustrated in the drawings. The drawings are a flow chart of a method of determining an integrity risk and a block diagram of an apparatus for determining an integrity risk according to an embodiment of the present invention.
In the following, identical and/or functionally identical elements may be provided with the same reference symbols. The absolute values and dimensional specifications are only examples and represent no restriction of the invention to such dimensions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The satellite reference system may be the Galileo System to whose integrity concept reference is made.